1. Field of the Invention
The present invention relates in a general manner to the resource control for satellite telecommunication networks using the asynchronous time-division technique ATM (Asynchronous Transfer Mode).
2. Description of the Prior Art
After the introduction of the services offered by the "narrow-band" integrated service digital networks, the network designers sensed the need to produce a more efficient network in the medium-term. This need was based on the following considerations: a "narrow-band" digital network does not enable complete integration of the existing or "futuristic" telecommunication services. The existing digital networks do not fully satisfy needs as regards data communications with very high bit rate, services generating a traffic flow of a highly sporadical nature, or in the long-term, video communications. The asynchronous time-division technique ATM was thus advocated to meet three requirements: digital technology, high bit rate, and flexibility in the routing of the different types of communications.
An international strategy for the development of "wide-band" integrated service digital networks (WB-ISDN), defined by the CCITT, is based on three fundamental principles:
an information transfer unit, called "cell", or also "packet", which is the basic element processed by all means constituting the network: terminals, multiplexers and switches. An ATM cell typically comprises 53 bytes, 48 being allocated to the transmission of information and 5 to the routing of the cell in the network, PA1 an asynchronous processing of the cells by the various means in the network, and PA1 a connection-oriented technique: a communication must be set up prior to all transfers of information. The mode selected is the "virtual-circuit" mode for the transmission of the cells throught the nodes of the network, of a type comparable to X.25 networks. PA1 the random technique aimed at generalizing the resources but consequently entailing fluctuations in the cell transmission delay far in excess of the limits specified for conversational services such as the telephone service, and PA1 the access-by-reservation technique. PA1 the decision to accept or refuse a given sporadic connection to be set up subsequent to a sporadic connection request in the signalling sub-frame by an earth station, the decision being determined as a function of an algorithm in which the average lengths of the bursts and sporadicnesses are considered as predetermined constants and which depends on maximum bit rates and average bit rates of the connections in process and of said given connection, the maximum bit rate and said average bit rate relating to said given sporadic connection being included in the connection request, PA1 subsequent to acceptance of the sporadic connection request, the transmission of a resource allocation request in the signalling sub-frame from the earth station to the central station in response to the beginning of the reception and storing of each of the bursts of the given connection in the earth station, and PA1 the anticipated reservation of a resource in the form of a time interval in the data sub-frame for the burst by the central station as soon as a time interval of the data sub-frame is released so that the earth station can begin transmitting the burst while continuing to store the burst.
The conventional networks using the circuit mode, which is the main component of the "narrow-band" integrated service digital network, solve all of their resource control problems at admission of calls in the network: once a call has been set up, a fixed bit rate resource is reserved for a user. The circuit mode would not of course suit the WB-ISDN network for obvious reasons of under-utilization of the transmission means due to the highly sporadic nature of certain communications.
Conversely, the ATM technique is based upon packet mode communications and therefore on operation in queues. Nevertheless, contrary to the techniques usually used for congestion control in conventional packet networks, an oversizing of the network or a step-by-step control of the traffic between nodes of the network is inconceivable for a wide-band network in which the bit rates can reach a hundred Mb/s. The control mechanisms substantially penalize the transmission times and prove inefficient. A solution consists in posing the problem backwards: if the congestion of the network cannot be controlled (deterioration of transmission times), it must be avoided by means of resource allocating mechanisms when accesses to the network are requested. But this prior condition imposes evaluation of the resource to be reserved as a function of the service: it is as easy to determine a resource to be reserved for a request for a constant or variable bit rate service, as it is difficult to define a resource to be reserved for highly-sporadicalness services.
When a source-user transmits a connection request, the request must contain the address of the destination but also the capacity required to route a traffic associated with the request. This request is routed within the network, from one node to another, according to routes determined by a routing algorithm. The call is accepted if there exists at least one route between the source-user and the destination such that each internodal link of said route has sufficient capacity to satisfy the transmission capacity required by the user. The problem has been stated: what bit rate capacity has to be required to route traffics having highly sporadic flows?
The complete generalization of the capacity of the network makes it difficult to provide an answer, insofar as the connections interact with one another.
Modification of the existing telephone network infrastructure to produce a wide-band digital network cannot be envisaged in the medium-term. These reasons are notably based on financial considerations, the means to be implemented to install a terrestrial wide-band digital network being considerable. In order to get around this limitation, a more flexible and less costly natural solution consists in using satellite wide-band networks. Nevertheless, the problem of resource control remains set in equivalent terms, in this case for access to a satellite channel.
The time-division share of a satellite channel between plural stations has been mainly envisaged according to two techniques:
According to the latter technique, contrary to the random access technique in which a connection implicitly entails a "creation" of a time interval by synchronization, frames are periodically allocated to the communications. A frame format based on the TDMA principle and used in the TDMA-reservation protocol is shown in FIG. 2. A frame T.sub.r is divided into a data sub-frame STD.sub.r and a signalling sub-frame STS.sub.r. The sub-frames STD.sub.r and STS.sub.r are respectively allocated to the transmission of data and the transmission of resource allocation request/response messages.
According to certain embodiments, the sub-frame STS.sub.r has a length that is variable as a function of the bit rate. For an earth station ST.sub.i sending a resource allocation request, the TDMA reservation protocol consist in the transmission of a reservation message in an assigned interval ITS.sub.i of a signalling sub-frame STS.sub.r after complete reception of a burst of cells generated by a service of a sporadic nature of a user station. A central station SC, receiving all the reservation messages coming from earth stations, allocates the required capacity to the earth station ST.sub.i when sufficient transmission capacity is available. This TDMA-reservation protocol advantageously informs the central station SC by means of the signalling sub-frame STS.sub.r with regard to a precise allocation request since the earth station only forwards its request after complete reception of the burst, and therefore after taking the length of the latter into account. Nevertheless, this "store and forward" management which consists in storing the whole burst of cells before processing an allocation request, is not satisfactory for services introducing very long bursts since such a management induces transmission delays which are unacceptable for certain services (interrogation of image data bases).